They are the same as the mm0-mm7 MMX registers and will be overwritten
by the assembly code if gcc doesn't know about the MMX registers.
Note: They're all added to the list of clobbered registers in all cases
and not only when __MMX__ is not defined just to make sure that no other
bugs happen with this code just because some compiler version gets things
wrong.
Fixes bug #614466.
This avoids the "bad register name `%dil'" compilation errors on 32bit where
because of 'r' gcc puts the value in a general purpose register and then tries
to access the lower part as %dil/%sil which is not existing on 32bit. 'q' requests
a-d registers
